The invention discloses a composite coagulant with high content of phosphorus for treating sewage of a river basin, and relates to the technical field of phosphorus-removing composite coagulants. Thecomposite coagulant comprises dilute sulfuric acid, a dephosphorization agent A, a dephosphorization agent B, an ammonia nitrogen removal agent A, an ammonia nitrogen removal agent B, water, calcium hydroxy phosphate, activated carbon particles and sodium peroxide, the content ratio of the dilute sulfuric acid, the dephosphorization agent A, the dephosphorization agent B, the ammonia nitrogen removal agent A, the ammonia nitrogen removal agent B and water is 2: 1: 1: 1: 1: 4, and the content ratio of the calcium hydroxy phosphate, the activated carbon particles and the sodium peroxide is 1: 2:1. By the composite coagulant with the high content of the phosphorus for treating the sewage of the river basin, biological phosphorus removal and chemical phosphorus removal are fused organically,moreover, by chemical phosphorus removal, on the one hand, part of phosphorus substances in wastewater can be consumed through chemical reaction, on the other hand, phosphorus can be acidized in cooperation with a dilute sulfuric acid solution to generate phosphoric acid ions, and abundant phosphoric acid ions are provided for biological phosphorus removal.